I often thought that the Island/Jacob had something to do with his cancer. Originally I thought it took the cancer from Juliette's sister and stuck it in him, but maybe not. Possible that the Island is fed up with Ben (as he has turned out to not really be "one of the good guys" as he so often proclaims.) He has been acting righteous and devine- something I'm sure would bother the Island/Jacob. Ben realized that the Island/Jacob had intensions with Locke (and possibly Hurley and Rose) and that horrifies Ben- could he be losing his stance as the token leader? Could the cycle continue and dump him on his fanny? But then again, why send Jack, a spinal surgeon, if it wanted Ben gone- unless if that were by some power other than the Island's doing.

Regardless- I have always felt there was some kind of power struggle between Ben and the Island/Jacob. Ben is always put up against hurdles, and thus far has been able to manipulate his way past them. It reminds me of the power struggle between Locke and Ben, unfortunately in the latter situation, Ben always has the upperhand.